A sheet of steel 1.5 mm thick has nitrogen (N2) atmospheres on both sides at 1200°C and is permitted to achieve steady-state diffusion condition. The diffusion coefficient for N2 in steel at this temperature is 6 ´ 10-11 m2 /s, and the diffusion flux is found to be 1.2 ´ 10-7 kg/m2 -s. Also, it is known that the concentration of N2 in the steel at the high-pressure surface is 4 kg/m3 . How far into the sheet from the high-pressure side will the concentration be 2.0 kg/m3 ? Assume a linear concentration profile.

Along 8-cm diameter steam pipe whose external surface temperature is 900c connects two buildings. the pipe is exposed to ambient air at 70c with a wind speed of 50 km/hr blowing across the pipe. determine the heat loss from the pipe per unit length. (b) air at 500c enters a section of a rectangular duct (15 cm x 20 cm) at an average velocity of 7 m/s. if the walls of the duct are maintained at 100c. a) the length of the tube for an exit temperature of the air to be 40 0c. b)the rate of heat transfer from the air. c) the fan power needed to overcome the pressure drop in this section of the duct.